We had been using just the raw_info from the kernel to generate the attrs.
That meant that we'd need to keep track of which events we had seen before
(hash table, though since we only ever had one or two events, that might
have been overkill), and we lost information from the original event
selection.

Now we use the perf_eventsel pointer as the raw_info / user_data, and we
can just emit an attr based on what was originally submitted. That will
also allow us to support some of the generic hardware events in the future.